An important aspect of portable electronic devices is the provision, more particular generation and maintenance, of a regulated voltage from an unregulated voltage source, for example a battery. For the generation and maintenance of the required regulated voltage a voltage regulator is used, which is often called DC/DC-converter. Moreover, since the voltage provided by a battery varies in a relative broad range from fully charged status up to the its discharged status, a voltage regulator is required which is able to provide a certain output voltage from a range of input voltages, which comprises higher as well as lower voltage values as the certain output voltage.
Basically, a common implementation of a voltage regulator or voltage regulating device is a switching regulator, which, generally, is a circuit that uses an energy-storage element, such as an inductor or a coil, to transfer energy from an unregulated power source, for instance the afore mentioned battery, to a load in discrete pulses, which are sometimes also referred to as bursts. By feedback circuitry the energy transfer process can be controlled for maintaining a constant output voltage at a load connected to the output of the voltage-regulating device.
The family of switching voltage regulators comprises various types, which are commonly used today in portable electronic devices. One example is a buck converter, which is an inductor based voltage regulator used to step-down (buck) the unregulated voltage source. In contrast hereto, a boost converter is an inductor based voltage regulator used to step-up (boost) the unregulated voltage source. Accordingly, a buck-boost converter is required to provide a regulated output voltage form a source of which the voltage varies from being higher over being nearly the same to being lower. The buck-boost converter provides a regulated output over large variations in the unregulated voltage source, but tends to be less efficient than the buck or boost converter.
As disclosed in WO 2006/018772, one approach to improve such a DC/DC buck-boost or up/down, respectively, converter is to have two operating modes: a so-called pulse width modulated (PWM) operation mode and a pulse frequency modulated (PFM) operation mode. Basically, the PWM operation mode comprises current pulses with constant frequency and there is a continuous current. The PWM operation mode is preferred when higher currents are required from the current source at the input of the converter to the output of the converter. The PFM operation mode comprises pulses with variable frequency and there is a discontinuous current. The PFM operation mode is mainly used for low power loads that are when small or less current is to be carried form the current source to the output of the converter. In such cases the PFM operation mode is more efficient as the PWM operation mode due to the reduction of the switching currents needed for the switching elements in the converter.
In the PWM mode, in principle, a control signal, e.g. having a waveform of square wave, is provided to the control terminal of at least one switching element to control its ON/OFF or “conducting/no conducting” states, respectively. By increasing the average ON time of the switching device the output voltage may be increased, vice versa. Accordingly, modulating the duty cycle of the square wave control signal regulates the output voltage.
In the PFM mode, basically, the switching frequency of the control signal is changed in order to keep the output voltage Vout constant. In principle, an oscillator and a driver circuit, which generates the corresponding control signal, for example a rectangular signal, supplied to the control terminal of the at least one switching element, may be used to control the switched operating mode. The PFM mode of the DC/DC converter provides better efficiency at small output current levels that does the above PWM operation mode. Firstly, the PFM operation mode requires less turn-on transitions to maintain a constant output voltage than the PWM operation mode does, which results in a lower gate-drive power dissipation of the switching transistor. Further, the PFM operation mode can be achieved with a much simpler control circuit having fewer components; the power dissipation in a control loop of the PFM operation mode is less than that of the control loop of the PWM operation mode. However, when the output current reaches a moderate level and higher, the PFM operation mode of voltage regulation becomes impractical, since the maximum output current available from the PFM operation mode is generally much less than that available from the PWM operation mode.
WO 2006/018772 discloses an approach to determine the timing and shape of a PFM operation mode pulses in an automatic up/down converter. Basically, a look-up table containing a list of ON-times for the UP-phase and the FORWARD-phase is provided, wherein the length of a PFM mode pulse is measured and if it is too long, shorter ON-times are chosen. When the converter operates as a down-converter, the ON-time for the UP-phase is set to zero. When the converter operates as an up-converter, the ON-time for the DOWN-phase is set to zero. For instance, in a range of approximately 90% of Vbat up to 110% of Vbat for Vout, both an UP-phase and a DOWN-phase exist. However, one drawback in this automatic up/down-converter is the need for a high-frequency clock for the control circuit of the converter. In particular, the clock needs to be approximately 8 times as high in frequency as the respective PFM frequency. As a result, when the desired PFM frequency increases, the required clock frequency increases to unacceptable levels due to the fact that high clock frequencies mean large dissipation in the clock generator, which is not desired for portable applications. Further, it also becomes more difficult to obtain the required accuracy.